Proteins are different based on number and type of contained in their polypeptide chain(s).

The correct answer would be amino acids.

The polypeptide chain is composed of different types of an amino acid linked through peptide bonds.

The number and sequence of amino acids determine the primary, secondary and tertiary structure of the proteins.

In some protein, the quarternary structure is also present.

Ultimately, tertiary and quarternary structures then determine the function and efficiency of the protein.

For example, the conformation of the active site in enzymes (proteins) determines the substrate that can bind to that enzyme.